Conventional manufacturing procedures for ceiling trims comprising heat-stable adhesives as part of the structure consist in heat forming in a hot thermoforming mould that is necessary to cure the heat-stable adhesive by the combined action of temperature and pressure.
Said curing consists in an irreversible chemical reaction by which the adhesive hardens, producing a structure of polymer chains that cross link to form a three dimensional network.
In order for said chemical reaction to occur it is necessary to activate the adhesive. The heat provided by the mould during thermoforming once the adhesive has been activated, causes the curing reaction of the adhesive until it hardens, thus giving the ceiling its final shape.
In the particular case of heat-stable polyurethane adhesives water is usually used as an activating element, and it is normally applied on the adhesive before introducing the different layers that will form the ceiling trim inside the hot thermoforming mould.
Afterwards, during the thermoforming step, the combined application of pressure and temperature provided by the mould result in the curing of the heat-stable polyurethane adhesive by contact with the layers that form the ceiling trim with the surfaces of the hot mould.
During this step, the adhesive tends to move towards the surfaces of the hot mould until it reaches them, crossing through the layers that are contiguous to the surfaces of said mould due to the effect of the pressure exerted by the mould, which results in its adhesion to them once the adhesive has cured due to the heat provided through the surfaces of the hot mould.
Said adhesion on the one hand makes demoulding the trim more difficult during extraction, which can also result in defects in the appearance of the trim, and on the other hand it stains the thermoforming mould.
In order to prevent this problem, the use of demoulding agents is known, which act as a barrier between the adhesive that forms part of the layers of the trim and the surfaces of the mould.
These may consist, for example, in liquids demoulding agents that are applied by spraying over the mould surface, or they may be demoulding agents in the form of sheets that are sandwiched between the layers that will make up the trim and the surface of the mould, and which are removed once the trim has been thermoformed and can be reused for a new cycle.
Patent WO2012031190 shows an example that describes a manufacturing procedure for a ceiling trim that uses a demoulding sheet that prevents the adhesive from sticking to the mould surfaces.
However, the use of demoulding sheets mandates the addition of an extra component, which is especially expensive in fast manufacturing processes.
Furthermore, both the liquid demoulding agents applied on the mould surfaces and the reusable demoulding sheets produce stains on the decorative coating that makes up the visible side of the ceiling trim as a result of the degradation of said demoulding agents over time.
On the other hand, the degradation of said demoulding agents also results in the deforming of the ceiling trim during its extraction from the mould, since it is adhered to the surface of the mould in areas where the demoulding agents are not acting due to their degradation.
In view of the above, the object of the invention consists in a manufacturing procedure for ceiling trims for vehicles comprising at least a first coating sheet and at least a second coating sheet that extend along one of the sides forming the first sheet, joined via a heat-stable polyurethane adhesive layer wherein no additional demoulding agents are used to facilitate extraction of the ceiling trim formed within the mould, while at the same time maintaining a good appearance of the ceiling trim obtained.